The feet of an individual are subject to substantial impact forces during regular activities such as standing and walking. The impact forces are particularly strong during athletic activities such as running and jumping. Footwear which provides proper cushioning and support may prevent the feet from discomfort, soreness, fatigue and injury caused by such impact forces.
Numerous cushioning elements or inserts used in footwear to cushion a wear's foot have been developed over the years to absorb the impact forces. Many of such cushioning elements or inserts are made at least partially of compressible materials. For example, foam materials such as ethylene vinyl acetate (EVA) and polyurethane (PU) have been widely used in the footwear industry to form part of a sole, due to its advantageous properties such as light weight and isotropic cushioning property.
U.S. Pat. Nos. 5,092,060 and 5,369,896 to Frachey et al. disclose a sports shoe having an insert which includes a plurality of barrel-shaped elements that are elastically deformable under pressure and are enclosed in an airtight casing of a plastic material. The insert is arranged in a seat provided in a wedge in a position corresponding with the heel of the foot and is confined by the surrounding wall portion of the seat. However, the plurality of barrel-shaped elements may cause individual pressure points above each element, resulting in uneven pressure distribution. Furthermore, if a customer presses a side of the shoe by a thumb to check the pliability or elasticity of the shoe sole, which often happens in the process of selecting a pair of shoes to buy, the shoe may not satisfy the thumb test because it does not feel soft or pliable peripherally.
U.S. Pat. Nos. 4,874,640 and 5,235,715 to Donzis disclose a composite for absorbing and dispersing impacting forces. The composite includes a flexible plastic enclosure defining an internal cavity. The flexible enclosure is generally impermeable to air and capable of having its internal pressure changed. The composite further includes a foam core filling the cavity and retained within the cavity and adhered on substantially its entire external surface to the internal surface of the cavity. The cavity can be pressurized for higher impact absorbance. The method for producing the composites includes forming the outer enclosure by applying a solution/suspension of the prepolymer of controlled solids content to the core and also can include the step of preheating the core prior to coating it. To withstand the high inflation pressures in such air bladders, the foam core must be of a high strength which requires the use of a higher density foam. The higher the density of the foam, the less the air space available in the bladder. Consequently, the reduction in the amount of air in the bladder decreases the cushioning benefits provided by air. Furthermore, air bladders relying on elevated internal pressures to cushion are susceptible to punctures and leakage.
U.S. Pat. Nos. 5,343,639 and 5,353,523 to Kilgore et al. disclose a midsole for a shoe including one or more foam columns disposed between an upper and a lower plate. The foam columns may include grooves formed on the exterior surface. One or more elastic rings are disposed about the columns and are removably disposable in the grooves, allowing the stiffness of the columns to be adjusted. In a further embodiment, inflatable gas bladders are disposed in the hollow regions. The structure and manufacture of such midsoles are very complicated.
As described above, all of these constructions are not completely satisfactory. Furthermore, compressible materials such as foam in shoe soles tend to break down over time, especially notably when the shoe soles are frequently subject to abrasions and exposed to excessive heat and moisture, resulting in discoloration and mushy appearance of the soles, which are not only aesthetically undesirable, but also may affect their shock absorbing capacity. Therefore, there remains a need for a cushioning insert for footwear that has improved durability and pliability, while keeping all the advantageous properties of compressible materials and eliminating the drawbacks of the above described constructions.